The invention relates generally to electrical generators and, more particularly, to a control scheme for exciting an electrical generator having fractional-slot concentrated windings and rotor field windings.
The usage of electrical machines in various industries has continued to become more prevalent in numerous industrial, commercial, and transportation industries over time. There has been tremendous progress and great achievements in the field of power electronics and control techniques for such electrical machines, resulting in increased energy savings and control flexibility. Providing for such achievements has been the continued progress in computer technology that has resulted from digital technology. Digital technology has lead to very significant reductions in the size and cost of computers, allowing them to successfully replace old, bulky, and relatively expensive mechanical systems.
While the capability of digitally enhanced control systems and computers has progressed, the structure of the electrical machines used with such control systems has, for the most part, remained unchanged. For example, the large majority of fixed speed electrical generators, such as those used in power stations, are designed using distributed sinusoidal windings on the stator and a DC field or permanent magnets on the rotor. As shown in FIG. 1, a prior art electrical generator 2 may be equipped with integral-slot distributed stator windings 4 and permanent magnets 6 on the rotor 8. As an example, FIG. 1 illustrates a 24-slot, overlapping distributed arrangement of stator windings 4. In operation, the permanent magnets 6 of generator 2 create the magnetic field in the air gap between rotor 8 and stator windings 4, which rotates the rotor 8 and generates electrical energy in the stator windings 4.
Construction of electrical generators in accordance with the structure of generator 2 illustrated in FIG. 1, that implement distributed sinusoidal windings 4, are however subject to drawbacks in performance and costs associated therewith. For example, electrical generators that implement distributed sinusoidal windings suffer from a decreased efficiency due to electrical losses in the end windings. The end winding length contributes to increased resistance, thereby resulting in higher Ohmic losses that decrease the efficiency of the generator. The end winding length also requires implementation of a complex cooling system (e.g., liquid hydrogen cooling system), which leads to increased cooling cost in the electrical generator. Furthermore, the permanent magnets limit power density, energy efficiency, operating temperature, life cycle, and reliability of the electrical generator.
In addition to increased operating costs, electrical generators such as shown in FIG. 1 that implement distributed sinusoidal windings and permanent magnets are also more costly to construct. For example, such electrical generators often include expensive thin stator laminations that are expensive to construct. Furthermore, the permanent magnets on the rotor used to create the air gap magnetic field are expensive compared to generators incorporating electromagnets or field windings.
Therefore, it would be desirable to design an electrical generator that can employ non-sinusoidal stator windings so as to reduce costs associated with production and operation thereof. It is further desired that a control scheme be provided for controlling electrical generators that employ non-sinusoidal stator windings that suppresses the effect of the additional harmonic components typically associated with non-sinusoidal windings, so as to minimize harmonics and maintain high power density and high efficiency in the generator.